It is well known in the communications industry that noise or disturbances in a communications channel can adversely affect the performance of data communications equipment (DCE) using the channel. For example, it is well known that dial-up voice-frequency modems are constrained by noise in the channel to operate at less than their full capacity. This is why dial-up frequency modems do not always operate at their advertised maximum data rates. The performance of xDSL modems is also adversely affected by noise. In accordance with the present invention, it has been determined that periodic disturbances in premise wiring may adversely affect data communications equipment at the customer premises. Periodic impulse noise at the 60 Hz rate, which may be produced by radiating sources such as light dimmers, vacuum cleaners, blenders, etc., adversely affects data communications equipment at the customer premises. While these impairments may not adversely affect dial-up modems to a great extent, it is believed that they significantly adversely affect high-speed modems, such as DSL modems, because these types of modems utilize higher frequencies where pickup from radiating sources in the 60 Hz range may be greater.
Many DSL line coding techniques, such as discrete multi-tone (DMT), asymmetric digital subscriber line (DMT-ADSL), carrier less amplitude and phase modulation ADSL (CAP-ADSL), integrated services digital network (ISDN), and 2B1Q high-bit rate DSL (2B1Q-HDSL), do not attempt to communicate with general premise wiring but either (1) use a POTS splitter or filter to prohibit premise wiring disturbances above approximately 20 KHz from coupling into the DSL modem or (2) terminate the connection to the subscriber line at a single DSL modem such that the DSL modem does not communicate with any premise wiring. Therefore, coupling of premise wiring disturbances with frequencies above 20 KHz into the DSL communications channels is not a major problem. With respect to dial-up modems, coupling of periodic disturbances above approximately 4 KHz is not a problem because dial-up modems which operate over general premise wiring generally do not operate above 4 KHz. However, some DSL systems communicate over general premise wiring without a POTS splitter. In these cases, periodic disturbances coupled into the general premise wiring can significantly degrade performance.
If periodic disturbances contain only a relatively small number of frequency components within the data communication bandwidth, e.g., 1 to 15 disturbing frequency components, techniques such as precoding and data-feedback-equalization (DFE) can be used to improve performance. However, if there are many disturbing components within the data communication bandwidth, preceding and DFE techniques are impractical and can lead to other problems, such as long signal delays. With respect to periodic impulse disturbances in the 60 Hz range, these impulses imply a spectral content with energy at frequencies separated by 60 Hz, which is far too fine a separation to be dealt with by standard preceding or DFE techniques. At this frequency, the impulse period is about 16.67 milliseconds, which is too short to consider transmitting only between the impulses, especially since the impulses are likely to be spread over perhaps a millisecond.
Accordingly, a need exists for a method and apparatus which measures periodic impulse disturbances at the customer premises and which produces a counter measure which overcomes the disturbances to improve performance of data communications equipment located at the customer premises. Furthermore, in accordance with the present invention, it has been determined that canceling the periodic disturbance at one customer premise modem receiver will effectively cancel the disturbance on all customer premise modem receivers, due to the very close physical proximity of the modems relative to the frequency band of communication. Also, since both the periodic disturbance at issue and the transmitted countermeasure are generated nearly concurrently at the premise modem, the cancellation will be equally effective at the remote end of the subscriber loop, i.e., at the central office.